hysics are so inextricably interwoven with it that it is necessary,
in an elementary exposition, to assume its existence. The modern
view will be explained later in the article on Einstein's Theory.

While the world wondered at these marvels, the men of science were
eagerly following up the new clue to the mystery of matter which was
exercising the mind of Crookes and other investigators. In 1896
Becquerel brought us to the threshold of the great discovery.

Certain substances are phosphorescent--they become luminous after they
have been exposed to sunlight for some time, and Becquerel was trying to
find if any of these substances give rise to X-rays. One day he chose a
salt of the metal uranium. He was going to see if, after exposing it to
sunlight, he could photograph a cross with it through an opaque
substance. He wrapped it up and laid it aside, to wait for the sun, but
he found the uranium salt did not wait for the sun. Some strong
radiation from it went through the opaque covering and made an
impression of the cross upon the plate underneath. Light or darkness was
immaterial. The mysterious rays streamed night and day from the salt.
This was something new. Here was a substance which appeared to be
producing X-rays; the rays emitted by uranium would penetrate the same
opaque substances as the X-rays discovered by Roentgen.

Discovery of Radium

Now, at the same time as many other investigators, Professor Curie and
his Polish wife took up the search. They decided to find out whether
the emission came from the uranium itself or _from something associated
with it_, and for this purpose they made a chemical analysis of great
quantities of minerals. They found a certain kind of pitchblende which
was very active, and they analysed tons of it, concentrating always on
the radiant element in it. After a time, as they successively worked out
the non-radiant matter, the stuff began to glow. In the end they
extracted from eight tons of pitchblende about half a teaspoonful of
something _that was a million times more radiant than uranium_. There
was only one name for it--Radium.

That was the starting-point of the new development of physics and
chemistry. From every laboratory in the world came a cry for radium
salts (as pure radium was too precious), and hundreds of brilliant
workers fastened on the new element. The inquiry was broadened, and, as
year followed year, one substance after another was found to possess